Radiation hardened bipolar junction transistor

ABSTRACT

A method of forming bipolar junction devices, including forming a mask to expose the total surface of the emitter region and adjoining portions of the surface of the base region. A first dielectric layer is formed over the exposed surfaces. A field plate layer is formed on the first dielectric layer juxtaposed on at least the total surface of the emitter region and adjoining portions of the surface of the base region. A portion of the field plate layer is removed to expose a first portion of the emitter surface. A second dielectric layer is formed over the field plate layer and the exposed portion of the emitter. A portion of the second dielectric layer is removed to expose the first portion of the emitter surface and adjoining portions of the field plate layer. A common contact is made to the exposed first portion of the emitter surface and the adjoining portions of the field plate layer. In another embodiment, the field plate and emitter contact are formed simultaneously.

BACKGROUND AND SUMMARY OF THE DISCLOSURE

The present disclosure is related to a bipolar junction transistor andmore specifically to radiation hard bipolar junction transistors inintegrated circuits and their method of manufacture.

Bipolar junction transistors (BJTs) are sensitive to ionizing radiation;such radiation causes junction leakage and gain degradation. Ionizingradiation causes hole-electron pair generation in the oxide layers overthe device junctions. Electrons thus generated have very high mobilityin oxide and drift toward the device's terminals in picoseconds. Holes,on the other hand, have low mobility and tend to be trapped in theoxide. This causes a trapped positive charge in the oxide, which in turnleads to changes in the depletion layers and surface concentrations inthe underlying device structure.

The rates of hole trapping in the oxide vary. The trapping ratedetermines the device's response to radiation. The trapping rate isinfluenced by factors such as dose rate, dielectric film stress, oxidedefects, applied electric field and passivation layer characteristics.In particular, low dose rate irradiation has been found to degrade BJTsmuch more rapidly than the moderate dose rates used in qualificationtesting. See, for example:

-   (a) E. W. Enlow, R. L. Pease, W. E. Combs, R. D. Schrimpf and R. N.    Nowlin, “Response of advanced bipolar processes to ionizing    radiation,” IEEE Trans. Nuc. Sci., vol. 38, p. 3049, December 1991;-   (b) A. H. Johnston, C. I. Lee and B. G. Rax, “Enhanced damage in    bipolar devices at low dose rates: Effects at very low dose rates,”    IEEE Trans. Nuc. Sci., vol. 43, p. 1342, December 1996; and-   (c) J. L. Titus et al., “Enhanced Low Dose Rate Sensitivity (ELDRS)    of Linear Circuits in a Space Environment,” IEEE Trans. Nuc. Sci.,    vol. 46, p. 1608, December 1999.

This is a somewhat counterintuitive result but has turned into a majorissue, as these low dose rates are exactly what is actually encounteredin space. In the context of this disclosure, a moderate “qualification”dose rate is in the range of 50–300 rad(Si)/s, while a low dose rate isto the order of 0.01 rad(Si)/s. The reason for using the “qualification”dose rate is a logistical one—a lot qualification to a 300 rad(Si)specification takes an afternoon at 300 rad(Si)/s, and three months at0.01 rad(Si)/s.

The present disclosure addresses the enhanced sensitivity of BJTs to thelow dose rate environment. Radiation sensitivity of the BJT involvesinversion of the base surface near the emitter leading to emitter-basejunction spreading at the Si/SiO₂ interface and higher base current dueto increased surface recombination. The hardness of the BJT depends onthe hardness of this parasitic MOSFET-like region to inversion to avoidemitter depletion spreading.

The enhanced low dose rate sensitivity (ELDRS) phenomenon is believed torevolve around higher charge trapping rates at the lower dose rates. Thephenomenon is not very repeatable, with substantially different resultsdepending on such factors as processing, fabrication facility and biasduring irradiation. Transistor-level radiation testing at IntersilCorporation has shown very severe degradation of the low-current gain ofthe transistor after low dose rate irradiation. FIG. 1 shows arepresentative figure, taken from unpublished Intersil Corporation data.The figure shows forward beta versus collector current for an NPN deviceirradiated at 10 mrad(Si)/s at room temperature. The upper curverepresents the pre-irradiation data, while the three lower curvesrepresent the 25 krad(Si), 50 krad(Si) and 100 krad(Si) data,respectively. This data indicates that designing parts to be hard inthis environment would involve some difficult tradeoffs with powerdissipation, as the transistors in a hardened design would need to beoperated at collector current levels of one or two orders of magnitudehigher than those found in the current designs.

Attempts at hardening commercial processes have included changes to oroutright elimination of the passivation layer. See, for example, R. L.Pease et al., “Total-Dose Hardening of a Bipolar-Voltage Comparator,”IEEE Trans. Nuc. Sci., vol. 49, p. 3180, December 2002. Investigation ofthe effects of dielectric film stress is described in a paper by M. R.Shaneyfelt et al., ‘Impact of Passivation Layers on EnhancedLow-Dose-Rate Sensitivity and Pre-Irradiation Elevated-TemperatureStress Effects in Bipolar Linear IC's,’ IEEE Trans. Nuc. Sci., vol. 49,p. 3171, December 2002. None of these methods have been entirelysuccessful. One method of forming a field plate layer bridging theemitter-base junction and separated therefrom by a thin oxide layer isshown in U.S. Pat. No. 4,590,664.

Another method for hardening junction bipolar transistors in integratedcircuits by changing the basic device structure is proposed in thisdisclosure. The method includes forming a base region in a surface of acollector region of a substrate and an emitter region in the surface ofthe base region. A mask is applied exposing the total surface of theemitter region and adjoining portions of the surface of the base region.A first dielectric layer is formed over the exposed surfaces. A fieldplate layer is formed on the first dielectric layer juxtaposed on atleast the total surface of the emitter region and adjoining portions ofthe surface of the base region. A portion of the field plate layer isremoved to expose a first portion of the emitter surface. A seconddielectric layer is formed over the field plate layer and the exposedportion of the emitter. A portion of the first and second dielectriclayers is removed to expose the first portion of the emitter surface andadjoining portions of the field plate layer. A common contact is made tothe exposed first portion of the emitter surface and the adjoiningportions of the field plate layer.

The resulting integrated circuit includes at least one bipolar junctiontransistor having a base region in a surface of a collector region andan emitter region in the surface of the base region. A field plate layeris juxtaposed on and separated by a first dielectric layer fromadjoining portions of the surface of the base and emitter regions. Acommon contact is to a first portion of the emitter surface and thejuxtaposed adjoining portions of the field plate layer.

Another method of forming a bipolar junction device in an integratedcircuit comprises applying a mask exposing the total surface of theemitter region and adjoining portions of the surface of the base regionand forming a first dielectric layer over the exposed surfaces. Aportion of the first dielectric layer is removed to expose a firstportion of the emitter surface. A common contact and field plate areformed to the exposed first portion of the emitter surface and on thefirst dielectric layer over the emitter-base junction.

In another embodiment, the bipolar junction transistor includes a commoncontact and field plate layer having a field plate portion juxtaposed onand separated by a first thin dielectric layer from adjoining portionsof the surface of the base and emitter regions; and a contact portionextending through the first dielectric layer to a first portion of theemitter surface.

These and other aspects of the present disclosure will become apparentfrom the following detailed description of the disclosure, whenconsidered in conjunction with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of gain versus collector current as a function of lowdose rate irradiation.

FIGS. 2–8 are cross-sectional views of an integrated circuit at variousstages of fabrication, according to the principles of the presentdisclosure.

FIGS. 9–11 are cross-sectional views of an integrated circuit at variousstates of fabrication of another embodiment, according to the principlesof the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In prior art devices, the thick, unhardened oxide or dielectric over theemitter-base junctions of the BJT traps positive charge efficiently. Ifthis dielectric could be made thinner, the volume available for chargetrapping would be reduced, and the device hardness would improve. Thethin dielectric may also be hardened by several well-known methods,which would reduce charge trapping further. Additionally, a field platecovering the junction would positively control the potential at thesilicon surface. As most BiMOS processes have a thin gate oxide and agate polycrystalline layer, the revised BJT structures can be builtreadily and without any additional masking levels. An example of aconventional, non-hardened thin oxide is that used for on-chip MOScapacitors.

This structure results in a field plate over the emitter-base junction,with thin oxide under the field plate. The thin oxide may be hardened ornon-hardened, depending on the process being used. The field plate isconnected to the emitter metallization by an oversize contact aperture.In this way, the field plate is always tied at one Vbe (about 700 mV)above or below the base, which limits the electric field at the surface.Additionally, the polarity of the applied field plate voltage will tendto enhance the surface concentration of the BJT base.

This field plate/thin dielectric structure improves the radiationhardness of the BJT structure because:

-   -   (a) the dielectric is thin, so the total oxide volume available        for hole trapping is minimized;    -   (b) a hardened dielectric, if used, has controlled low hole        trapping characteristics; and    -   (c) the field plate controls the electric field near the        surface.

The method for forming the doped regions may vary from the presentprocess. It is only used as an example. Also, the process may includebipolar junction transistors only, without any insulated gate fieldeffect transistors.

As illustrated in FIG. 2, the substrate includes two N− regions 10 and aP− region 12. One of the N− regions 10 and the P− region 12 each formthe collector region of the NPN and PNP bipolar transistors. The otherN− region 10 is the region in which the N− channel insulated gate fieldeffect transistor (IGFET) is formed. P base region 14 is formed in N−collector region 10, N base region 16 is formed in P− collector region12, and a P body portion 18 is formed in N− substrate 10. During thediffusion process, a dielectric layer 20 (for example, silicon dioxide)is deposited on the surface of the substrate.

As illustrated in FIG. 3, a photoresist process is performed to formopening 24 in the dielectric layer 20. P+ impurities are then introducedthrough opening 24 to form emitter region 34 in base 16. Also, asillustrated in FIG. 3, a photoresist process is performed to formopenings 22, 26, 28 in the dielectric layer 20. N+ impurities are thenintroduced through openings 22, 26, 28 to form the emitter region 32 inbase 14 and the source and drain regions 36, 38 in body 18. As shown inFIG. 4, the dielectric layer 20 is then further etched to define opening42 exposing the total surface of emitter 32 and adjoining surface areasof the base 14. Similarly, opening 44 is formed to expose the totalsurface of emitter 34 and adjoining regions of the base 16. The oxidelayer 20 and those portions of the oxide which are formed over thesource and drain regions 36, 38 are further processed to form an opening46 in the channel region of the body 18 between the source and drainregions 36, 38. As can be seen from FIG. 4, this masking step is thesame masking step for the gate oxide of the IGFET device. It would notbe an additional step in integrated circuit processing. If theintegrated circuit does not include insulated gate field effecttransistors or other devices using a thin oxide, such as MOS capacitors,this would be an extra step in the process.

A thermally grown thin oxide layer 48 is then formed in the openings 42,44, 46. This thin oxide is preferably a radiation hard oxide, although anon-hardened process may be used as well. The hardened thin oxide may bemade radiation-hard by several well-known techniques, such ascontrolling the oxide growth ambient and temperature. As with theprevious step, the formation of the thin oxide for bipolar transistorsis the same process step to form the oxide used to form the gateinsulator of the insulated gate field effect transistor. If the bipolarprocess was performed without insulated gate field effect transistors,other radiation hard or non-radiation hard dielectric layers, such ascapacitor dielectrics, may be used.

Next, as illustrated in FIG. 5, a layer of conductive material (for thisexample, polycrystalline silicon or metal) is applied and patternedusing photoresist and etch to form field plate 50, which is juxtaposedover the total surface of emitter 32 and adjoining portions of the base14. A similar field plate 52 extends over the total surface of theemitter 34 and adjoining portions of the base 16. The plates 50, 52 havealso been patterned during the same process to form an opening 56 toexpose a portion of emitter 32 and an opening 58 to expose a portion ofemitter 34. As will be discussed below with respect to the emittercontact, this allows a single contact to field plates 50, 52 and theemitter regions 32, 34, respectively. The layer 54 also extends acrossthe channel region of P body region 18 between source and drain regions36 and 38, forming the gate of the IGFET device.

Next, the integrated circuit is covered by an inter-level dielectric 60.This is done by deposition and densification of, for example, silicondioxide. This process is well known and is illustrated in FIG. 6. Aphotoresist and etch process is conducted to form openings in theinter-level dielectric 60 and the dielectric layer 20 to the regions ofthe integrated circuit. For purposes of clarity, only those contacts tothe emitter/field plates and gates are shown in FIGS. 7 and 8. Anopening 62 is formed in layer 60 exposing the portion of field plate 50and a portion of the emitter 32 through an opening 56 in the field plate50 at opening 63 in the dielectric layer 20. An opening 64 in thedielectric layer 60 exposes a portion of the field plate 52 and aportion of the emitter 34 through an opening 58 in the field plate 52 atopening 65 in dielectric layer 20. Opening 66 in dielectric layer 60exposes a portion of the gate 54 of the insulated field effecttransistor. All of these are shown in FIG. 7.

The interconnect layer or material (for example, metal) is applied andthen delineated to form the contacts with the interconnect. Asillustrated in FIG. 8, contact 72 extends down through dielectric layer60 to come into contact with the field plate 50 and the emitter 32. Itforms a common contact. Contact 74 extends through dielectric layer 60to form a common contact with field plate 52 and emitter 34. Contact 76extends through dielectric layer 60 and makes direct contact with thegate 54.

As previously discussed, all of the steps used to form the field plates50, 52 extending from the emitters 32, 34 onto adjacent portions of thebases 14, 16 and to form a common contact to the field plate and theemitter regions are all part of a process to form insulated gate fieldeffect transistors. Thus, there are no additional process steps includedin those cases where the integrated circuit process includes insulatedgate field effect transistors. Although not shown, the integratedcircuit may also include self aligned IGFETs. The process would be thesame, except the size of the gate 54 would be smaller and there would beadditional steps between FIGS. 5 and 6 to form the self aligned regionsof the source and drain.

If insulated gate field effect transistors are not included, theadditional steps are only that of FIGS. 4 and 5—namely, the openings 42,44 to form the emitter base junction, the application of the dielectriclayer 48, and applying the field plates 50, 52. The subsequent step ofinter-level dielectric deposition of FIG. 6 would not be needed and thecontact and interconnect formation steps of FIGS. 7 and 8 are part of astandard bipolar transistor process. Such a process is illustrated inFIGS. 9–11 only for the NPN transistor, but the same steps would be usedfor the PNP transistor. These steps correspond to the steps of FIGS. 5,7 and 8. The thin dielectric layer 48 is formed in opening 42, as shownin FIG. 9. A photoresist and etch process is conducted to form opening63 in the dielectric layer 20, as shown in FIG. 10. The interconnectlayer or material (for example, metal) is applied and then delineated toform the contacts with the interconnect. As illustrated in FIG. 11,contact 72 extends down through dielectric layer 20 to come into contactwith the emitter 32 and forms the combined field plate and emittercontact.

Although the present disclosure has been described and illustrated indetail, it is to be clearly understood that this is done by way ofillustration and example only and is not to be taken by way oflimitation. The scope of the present disclosure is to be limited only bythe terms of the appended claims.

1. A method of forming a bipolar junction device in an integratedcircuit comprising: forming a base region in a surface of a collectorregion of a substrate; forming an emitter region in the surface of thebase region; forming a mask exposing the total surface of the emitterregion and adjoining portions of the surface of the base region; forminga first dielectric layer over the exposed surfaces; forming a fieldplate layer on the first dielectric layer juxtaposed on at least thetotal surface of the emitter region and adjoining portions of thesurface of the base region; removing a portion of the field plate layerto expose a first portion of the emitter surface; forming a seconddielectric layer over the field plate layer and the exposed portion ofthe emitter; removing a portion of the first and second dielectriclayers to expose the first portion of the emitter surface and adjoiningportions of the field plate layer; and forming a common contact to theexposed first portion of the emitter surface and the adjoining portionsof the field plate layer.
 2. The method according to claim 1, whereinthe first two steps of forming the base and emitter regions are repeatedto form complementary bipolar transistors, and the remaining steps areperformed simultaneously for the complementary bipolar transistors. 3.The method according to claim 1, including steps which form source anddrain regions of at least one insulated gate field effect transistor ina channel region of the insulated gate field effect transistor; andwherein the steps which form the first dielectric layer and the fieldplate layer also form the gate insulator and the gate, respectively, ofthe insulated gate field effect transistor.
 4. The method according toclaim 3, wherein one of the steps which forms the base region and theemitter region is also used to form the source and drains of theinsulated gate field effect transistor.
 5. The method according to claim1, wherein the mask formed is a third dielectric layer on the surface ofthe substrate with an opening to expose the total surface of the emitterregion and adjoining portions of the surface of the base region.
 6. Themethod according to claim 1, wherein the field plate layer is apolycrystalline silicon, and the contact is metal.
 7. The methodaccording to claim 1, wherein the first dielectric layer is a radiationhard layer.
 8. The method according to claim 1, wherein the field platelayer and the contact are metal.
 9. The method according to claim 1,wherein the first dielectric layer is a non-radiation hard layer.
 10. Amethod of forming a bipolar junction device in an integrated circuitcomprising: forming a base region in a surface of a collector region ofa substrate; forming an emitter region in the surface of the baseregion; forming a mask exposing the total surface of the emitter regionand adjoining portions of the surface of the base region; forming afirst dielectric layer over the exposed surfaces; removing a portion ofthe first dielectric layer to expose a first portion of the emittersurface; and forming a common contact and field plate to the exposedfirst portion of the emitter surface and on the first dielectric layerover the emitter-base junction.